In the past decade, the environmental impact of the jet engine as employed in modern aircraft has been so severe as to require special legislation to require modification on both the state and federal level. The environmental problems consist of two extreme conditions, namely noise and air pollution. Through extensive research, it has been found that modifying the surface structure of the foil through which the air passes by incorporating a repetitive hole pattern in the skin would enhance the air flow to provide better combustion, thereby reducing or eliminating the visible air pollution; especially as important, such a structure would muffle the sound of the jet engine, thereby reducing the noise to an acceptable level.
It is the principle of the jet engine that cold air is taken in one end, compressed, mixed with fuel and burned, thereby expanding the ignition products and air which provides the thrust. It is quite obvious that the entire stream of gases must be confined by a structure to produce the thrust backward, which thrust provides the propulsion for the aircraft. It is the containment walls or screens with which this invention is concerned.
It is apparent that as the gases enter the engine, they are cold, and as they enter the compressor section, they begin to warm-up from the compression and after ignition they become very hot, of the nature of 2,000.degree. F. and higher. As the materials of the retaining walls must withstand the great change in temperature and chemical composition of the gases to which it is exposed, it has been found to be sound engineering practice to meet the various environmental changes within the engine by selecting different material capable of withstanding the environment. For instance, in the cool section of the intake, aluminum is most frequently used; as the temperature increases above 250.degree. F., it is generally acceptable to employ 18 Cr-8 Ni steels up to a temperature of 700.degree.-800.degree. F.; above this temperature, it is necessary to fabricate the structure from the Chrome-Nickel or Chrome-Nickel-Molybdenum super alloy types such as Inconel 625, Hastelloy B, Hayes Alloy 25, Waspalloy, or the like. The compositions of these super alloys is as follows:
______________________________________ Cr Ni Mo Co Fe Nb T1 W ______________________________________ Inconel 625 21.5 61.0 9.0 -- 2.5 3.6 Hastelloy B 1.0 57.8 30. 2.5 6.0 Haynes Alloy 25 20.0 10.0 53.5 15.0 Waspalloy 19.5 55.1 4.3 13.5 2.0 Rene 41 19.0 56.9 10.0 11.0 3.1 ______________________________________
As the iron content of the alloy decreases, the corrosion resistance and high-temperature strength increase but with the increase in these desirable characteristics comes a decrease in workability. In other words, it becomes more and more difficult to find economically acceptable techniques to perform the machining and forming function necessary to provide a useful structure.
As an example of such a desirable structure for reduction of noise, a hole pattern of 0.030" diameter holes on 0.100" centers may be required. In the cold section and even the intermediate temperature section of the jet engine, such a structure may be manufactured by punching holes in a skin of aluminum or 18-8 stainless steel employing a gang punch which will produce a row of holes across an entire 48" wide skin. Staggered patterns require 2 rows of punches, that is 480 holes, with one hit of the punch. This operation is a standard manufacturing technique and has been found economically acceptable. However, where the process was applied to punching the high temperature resistant super-alloys found in the hot section, it was found that the punches would not physically withstand the high hardness of the super alloys.
It is a characteristic of the super alloys to become extremely hard as they are physically deformed. Therefore, even though the sheet of material of super alloy composition would appear to be relatively soft in the annealed state, as soon as the punch struck the metal it would begin to harden. Initially, the punch would perforate the metal but as soon as it became slightly dull, the metal would harden beneath the punch and break. If a punch broke, and was not immediately repaired, the entire pattern would be disrupted, thereby reducing the effectiveness of the panel.
Drilling was out of the question because of the expense of producing only one or a small number of holes at a time, and the work-hardening characteristics of the super-alloy is even more apparent in a machining operation, causing extensive drill breakage. Therefore, there was no known method for producing holes in the super alloy other than photo-etching.
Methods of etching the super-alloys have been known for a number of years but because of the extreme chemical resistance of the alloys, only one etching composition is known to be effective, that is an aqua-regia-fluoride acid bath of which a characteristic composition is as follows:
______________________________________ At 140-160.degree. F. ______________________________________ 32.degree. Baume Muriatic Acid 9 Gal. 40.degree. Baume Nitric Acid 1 Gal. 70% Hydrofluoric Acid .2 Gal. Anhydrous FeCl.sub.3 to specific gravity 1.26 ______________________________________
This acid composition is extremely active. It has been found that synthetic rubber based compositions are effective resists to this composition for a limited time, and are useful in the chemical milling of the super-alloys using hand scribing techniques (see U.S. Pat. No. 2,739,047) but this method of masking is not acceptable for production of 0.030" holes necessary for screen manufacturing.
The only possible technique for masking is the art of photomasking as employed in the printed circuit field. However, there is no known photomask with sufficient chemical resistance to withstand the above described etching solution. It is in this area that applicant has discovered a process which is acceptable for etching holes in the super alloys with the accuracy of punching or photomasking, to produce a super-alloy screen by an economically feasible method.